M. Heydari-Malayeri - Paris Observatory

An → ionization front of → H II regions
whose expansion speed is comparable to the → sound speed
in the gas (~ 10 km/sec for hydrogen at
104 K). A D-type ionization front results from
→ R-type ionization front when its propagation speed decreases as
the volume of gas ahead of the ionization front grows. If front velocity is equal
to a lower limit (C12 / 2C2, where C1
and C2 are the sound speed ahead and behind the front respectively),
the front is called D critical.

A spherical → ionization front of
→ H II regions
that moves radially outward from the → exciting star at a velocity
much higher than → sound speed
in the surrounding cold neutral gas of uniform
density (ahead of the front). R-type ionization fronts corresponds to early evolution of
H II regions, and
will eventually transform into → D-type ionization fronts.
If the motion of the front is supersonic relative to the gas behind as well as
ahead of the front, the front is referred to as weak R. The strong R
front correspond to a large density increase across the front.

A burst of → X-rays observed toward
→ low-mass X-ray binary (LMXB)s. It is characterized by a
sharp increase in → luminosity, which lasts 1-10 s, followed
by the peak and a slow decrease, which can last from ~ 10s to 100s. Observationally,
X-ray bursts manifest as a bright peak of emission on top of the persistent emission
powered by → accretion. See also
→ Type II burst.

An → orbital migration of low-mass
→ planets in which no gap is created in the
→ protoplanetary disk. According to planetary models,
beyond a critical core mass for the forming planet, a gap in the protoplanetary
disk is created. The critical mass depends on the mass and
→ metallicity of the disk and
therefore it does not have a singular value, but has been shown to be between about
10-30 Earth masses. Compare with → Type II migration.

A → Type I supernova that presents a singly-ionized silicon
(Si II) absorption feature at 6150 Å near peak brightness. Type Ia SNe
are believed to result from mass → accretion to a
carbon-oxygen → white dwarf in a → close binary
system. When the white dwarf mass exceeds the → Chandrasekhar limit,
the → degenerate electron pressure can no
longer support the accumulated mass and the star collapses in a thermonuclear
explosion producing a supernova. The → peak luminosity
of SNe Ia is set by the radioactive decay chain
56Ni → 56Co → 56Fe,
and the observed photometric correlation between the peak luminosity and the
time-scale over which the → light curve decays from its maximum
is understood physically as having both the luminosity and
→ opacity
being set by the mass of 56Ni synthesized in the explosion.
Type Ia supernovae occur in all types of galaxies.
Type Ia SNe are used as → standard candles in determining
cosmological distances, after normalizing their light curves with the
→ Phillips relation.

A → Type I supernova that has neutral helium line (He I) at
5876 Å, and no strong silicon (Si II) absorption feature at 6150 Å.
Type Ib supernovae are believed to result from the evolution of
→ massive stars.

A burst of → X-rays observed toward
→ low-mass X-ray binary (LMXB)s and characterized by quick
succession of bursts with recurrence intervals as short as ~ 7 s. Type II X-ray bursts
look similar to → Type I bursts, but they are thought to be
related with spasmodic episodes of → accretion.

A supernova type whose spectrum contains hydrogen lines.
Compared with → Type I supernovae,
its → light curve has a broader peak at maximum and
dies away more rapidly. The magnitudes are smaller, ranging from
MV = -12 to -13.5, and the ejecta have lower
velocities (about 5,000 km/sec).
These supernovae, which result from the final evolution of
→ massive stars, have three main divisions:
→ Type II-P, → Type II-L,
and → Type II-n.

A → Type II supernova which reaches a plateau in its
→ light curve. The vast majority of Type II SNe are
characterized by a fast (few days) rise to a flat light curve, most pronounced in
the reddest optical bands, with a duration of 80-100 days. This plateau phase is
interpreted as the recession of the photosphere as the ejecta expand and cool.
The spectra of SNe II-P are typically dominated by strong
→ P Cygni profiles of hydrogen lines, as well as iron
absorption features (for a review, e.g., see Filippenko 1997, ARA&A 35, 309).